1. Field of the Invention
The invention relates generally to a method and apparatus for reducing delamination within a polycide structure. In particular, the invention relates to a method and apparatus for reducing separation at the interface between tungsten silicide and the doped polysilicon layers of a polycide. In the method of the invention a silicon wafer containing a doped polysilicon layer that has been deglazed and vapor etched in HF is returned to a cassette that is surrounded by an apparatus containing a continuous flow of nitrogen. The etched wafer remains in the nitrogen flow of the apparatus until the next processing step. Separation, presumably the result of surface oxidation on the polysilicon layer, appears to be reduced by the present invention.
2. Brief Description of the Prior Art
In the production of integrated circuits, it is common to form refractory metal polycides from refractory metal suicides and polysilicon, where the metal silicide has been formed by reaction of a refractory metal compound and silane. Titanium, tantalum, tungsten, molybdenum and cobalt, are some of the metals that may be used to form the polycide. The polycide layers are relatively high conductors used in complementary-metal-oxide-semiconductor (CMOS) devices. After a lithographic patterning step, polycides are used in materials for gate and interconnect formation.
Polycides are typically produced from tungsten or titanium silicide and doped polysilicon. A deglazed and vapor HF etched layer of doped polysilicon is deposited with metal silicide and then annealed at high temperatures to form the polycide.
An inspection of the wafer after annealing may reveal separation at the interface between the metal silicide and polysilicon. This "lifting" or "delamination" phenomenon has been observed by others (see Woo et. al., U.S. Pat. No. 5,422,311; Chittipeddi et. al., U.S. Pat. No. 5,147,820; and Huang et. al., U.S. Pat. No. 5,130,266). The portions of the wafer exhibiting separation cannot be processed any further, resulting in low product yields for the particular manufacturing process.
Solutions to the problem vary. Woo et. al. show a method for manufacturing a reduced resistivity conductor where tungsten silicide is deposited on a polysilicon film (etched to approximately 400 .ANG.). The metal silicide and polysilicon are then annealed to form a polycide. In addition to the polycide, a silicon layer forms at the interface between the polysilicon and silicide. As the silicon forms, adhesion between the two layers is increased preventing the lifting phenomenon.
Chittipeddi et. al., show a method for making a polycide by varying the conditions for the doped polysilicon depositions and decreasing the dopant concentration creating layers within the polysilicon. When the metal silicide layer is deposited onto the polysilicon, improved uniformity and planarity is obtained due to stress accommodations provided by the underlying layering. Chittipeddi et al. note that interfacial properties of the polysilicon were improved using this method.
Huang et. al., describe a method for making a lightly doped drain MOSFET device which overcomes the "peeling problem" of metal polycide layers on a polycide gate. Huang et. al. delete several steps of the normal manufacturing process, namely, thermal oxidation after polycide etching, densification after deposition of the silicon dioxide layer for spacer preparation, and silicon oxide capping of the refractory metal silicide layer after spacer formation. The modified process provides a non-peeling polycide gate.
Although these solutions have provided some relief from delamination, the solutions are specific to each individual process. Each instance of delamination must be addressed by tailoring the solution to the specific process. The present invention provides a general solution which may be used in numerous processes where polycides are formed.